Non-invasive imaging and analysis of targets is a valuable technique for acquiring information about systems or targets without undesirable side effects, such as damaging the target or system being analyzed. In the case of analyzing living entities, such as human tissue, undesirable side effects of invasive analysis include the risk of infection along with pain and discomfort associated with the invasive process.
Optical coherence tomography (OCT) is a technology for non-invasive imaging and analysis. There exists more than one OCT technique. Time Domain OCT (TD-OCT) typically uses a broadband optical source with a short coherence length, such as a super-luminescent diode (SLD), to probe and analyze or image a target. Multiple Reference OCT (MRO) is a version of TD-OCT that uses multiple reference signals. Another OCT technique is Fourier Domain OCT (FD-OCT). A version of Fourier Domain OCT, called Swept Source OCT (SS-OCT), typically uses a narrow band laser optical source whose frequency (or wavelength) is swept (or varied) over a broad wavelength range. In TD-OCT systems the bandwidth of the broadband optical source determines the depth resolution. In SS-OCT systems, depth the wavelength range over which the optical source is swept determines the depth resolution. Another variation of FD-OCT is spectral domain where the detection process separates wavelengths by means of a spectrometer.
TD-OCT technology operates by applying probe radiation from the optical source to the target and interferometrically combining back-scattered probe radiation from the target with reference radiation also derived from the optical source. The typical TD-OCT technique involves splitting the output beam into probe and reference beams, typically by means of a beam-splitter, such as a pellicle, a beam-splitter cube, or a fiber coupler. The probe beam is applied to the target. Light or radiation is scattered by the target, some of which is back-scattered to form a back-scattered probe beam, herein referred to as signal radiation.
The reference beam is typically reflected back to the beam-splitter by a mirror. Light scattered back from the target is combined with the reference beam, also referred to as reference radiation, by the beam-splitter to form co-propagating reference radiation and signal radiation. Because of the short coherence length, only light that is scattered from a depth within the target whose optical path length is substantially equal to the path length to the reference mirror can generate a meaningful interferometric signal.
Thus the interferometric signal provides a measurement of scattering properties at a particular depth within the target. In a conventional TD-OCT system, a measurement of the scattering values at various depths can be determined by varying the magnitude of the reference path length, typically by moving the reference mirror. In this manner the scattering value as a function of depth can be determined, producing a depth scan of the target.
Various techniques exist for varying the magnitude of the reference path length. Electro-mechanical voice coil actuators can have considerable scanning range, however, there are problems with maintaining stability or pointing accuracy of a reference mirror. Fiber based systems using fiber stretchers have speed limitations and have size and polarization issues. Rotating diffraction gratings can run at higher speeds, but are alignment sensitive and have size issues.
Piezo devices can achieve high speed scanning and can have high pointing accuracy, however to achieve a large scanning range requires expensive control systems and such systems have limited speed. A scanning method that effectively amplifies the scan range of a piezo device is described in the U.S. Pat. Nos. 7,526,329 and 7,751,862 referenced hereinabove.
The technique described in these publications uses multiple reference signals with increasing scan range and correspondingly increasing frequency interference signals. This scanning method can achieve large scan range at high speed with good pointing stability. The interference signals associated with the multiple reference signals are detected by a single detector as a complex signal consisting of the combined interference signals.
In swept source Fourier domain OCT systems depth scanning is accomplished by repeatedly sweeping the wavelength of the optical source. The wavelength range over which the optical source is swept determines the depth resolution. The period of the sweep repetition rate determines the period of the depth scans.
In addition to depth scanning, lateral scanning of a target is required for many imaging and analysis applications. Some conventional techniques for lateral scanning use stepper or linear motors to move the OCT scanning system. In some applications angular scanning is accomplished by electro-mechanical oscillating mirrors, typically referred to as galvo-scanners, which angularly deviate the probe beam.
Currently available OCT systems are bulky, weighty, complex and high cost. Currently available OCT systems have complex and bulky alignment and scanning sub-systems that result in physically large and costly systems. Moreover, in typical ophthalmic applications currently available OCT systems must be operated by a trained physician or technician. What is needed is a lightweight, robust, reliable monitoring device that is amenable to alignment by a layperson, and provides reliable and accurate measurements.
Furthermore, ophthalmic applications, such as retinal examination, often require the retina to be at a fixed orientation with respect to the OCT probe beam or with respect to the scanning region of the OCT probe beam. This process is also referred to as “fixation” of the eye. Currently available OCT systems that require fixation at locations other than the location being analyzed by the OCT beam also require a complex fixation mechanism.
Major causes of blindness are macular degeneration and diabetic retinopathy. Both of these conditions can benefit from timely medical intervention. People who are at risk of eye damage from these conditions need frequent monitoring because occurrence of an adverse situation (for example the growth of weak and leaky blood vessels), if not addressed in a timely manner, can cause irreversible damage to the retina leading inexorably to loss of vision.
Current practice involves monthly visits to a doctor. Many of these visits are wasteful if nothing has changed and in the case of a change, significant irreversible damage can occur within a month. Therefore, reducing the time between retinal measurements without being wasteful is advantageous.
In the retina of an eye, both the vascular system and the central nervous system are accessible for non-invasive analysis by an OCT system. This provides the opportunity to monitor for the onset or progression of a myriad of conditions, in addition to macular degeneration and diabetic retinopathy. Frequent monitoring of such conditions would be facilitated by a low cost system capable of making the required measurements without the aid of a trained professional.
There is therefore an unmet need for a low cost OCT system capable of making in-vivo OCT measurements of an eye, where such a system has automatic alignment, scanning and fixation mechanisms that do not need a trained operator and can preferably be operated by the subject him or herself What is also needed is a system that communicates scan results to a medical professional.